Platinum matrix composites



Oct. 13, 1970 TENSILE STRENGTH (psi x I03) F. HITTMAN PLATINUM MATRIXCOMPOSITES Filed Dec. 8, 1966 PLATINUM- I5 VOL.% ALUMINA WHISKERS(CALCULATED) PLATINUM (EXPERIMENTAL) TEMPERATURE C) INVENTOR FREDHITTMAN ORNEYS United States Patent 3,533,759 PLATINUM MATRIX COMPOSITESFred Hittrnan, Pikesville, Md., assiguor to Hittman Associates, Inc.,Baltimore, Md., a corporation of Maryland Filed Dec. 8, 1966, Ser. No.600,233 Int. Cl. B22f 7/08 US. Cl. 29132.2 4 Claims ABSTRACT OF THEDISCLOSURE The present invention relates to high strength, highductility and oxidation resistant platinum matrix, fiber or whiskerreinforced composite materials, and, more particularly, this inventionis directed to a class of platinum matrix structural materials having acombination of high strength, high ductility and oxidation resistance atboth room and elevated temperatures that are greatly superior incharacteristics to all presently known competitive materials.

It is known in the prior art to produce increased strength of metals foruse at elevated temperatures by producing such metals by dispersionhardening. Such metals include materials consisting of a metallic matrixcontaining a large number of small, inert, hard particles which may beobtained by precipitation of a constituent of the alloy or mechanicaldistribution of an insoluble component. In the dispersion hardenedmaterials, the load is borne by the metallic matrix, and the function ofthe dispersed phase is merely to impede the generation and motion ofdislocations within the matrix, thereby permitting it to bear a highstress without any deformation. in this material, the total content ofthe dispersed phase is generally one percent or less.

Platinum is indeed a potentially most attractive and desirable matrixmaterial, since it has a high melting point, a high ductility and easeof bonding to ceramics, and good oxidation resistance.

However, the prohibitive cost of platinum metal and its alloys and theavailability of other ostensibly similar metal matrix composites whichexhibit satisfactory strength and temperature characteristics hadheretofore served to stifie research and development along the lines ofhigh strength platinum matrix structural elements. In point of fact,silver composites strengthened by particles and whiskers are known toexhibit potentially useful tensile strengths of up to about 65,000pounds per square inch at 0 F. and up to about 43,000 pounds per squareinch at 1400 F. Compare Sutton et al., Metals Engineering Quarterly,February 1963, vol. 3, page 43.

It has now been surprisingly found that composite structural elementscomprising a platinum metal or a platinum metal alloy matrix can beproduced without a substantial increase in gross cost, the compositestructural elements unexpectedly exhibit tensile strengths in excess of135,000 pounds per square inch at 0 C. and in excess of 65,000 poundsper square inch at 1400 C.

Thus, an object of the present invention is that of providing the artwith reinforced structural elements of such greatly improved strengthcharacteristics at both room and elevated temperatures that manyhitherto impracticable applications for composite materials are renderedfeasible, both from economic and technical points of view.

It is yet another object of the instant invention to provide a highstrength, high ductility and oxidation resistant platinum matrixcomposite material for use at substantially elevated temperatures or atroom temperatures, and in which the cost of producing the material issubstantially reduced by increasing the fiber or whisker content tohigher levels and yet maintain, as well as increase, the

strength characteristics, the ductility and the oxidation resistancecharacteristics.

These and other objects, briefly stated, are accomplished according tothe present invention, by providing a composite product of platinum andceramic or other fibers or whiskers, or of platinum alloys and ceramicor other fibers or whiskers, comprising a metallic matrix of platinummetal or a platinum metal alloy such as platinum-rhodium and a dispersedfibrous or whisker component such as fibers or whiskers of A1 0particularly a-AI O The fiber or whisker content may range from onevolume percent to volume percent and may be randomly disposed ordisposed in a given orientation or in several given orientations,depending upon the particular application and use of the productcontemplated, and may be comprised of either continuous or discontinuousfibers or whiskers.

There is shown in the figure of drawing a characteristic curve for thebehavior of a platinum, 15 volume percent A1 0 whisker composite of thisinvention as a function of temperature.

In the fiber or whisker reinforced composites of the present invention,as well as in those of the prior art, the load is borne primarily by thefibers orwhiskers. The matrix serves to transmit the stress to thefibers or whiskers in shear over a relatively large surface area, andthereby avoid any significant stress concentrations that might fracturethe brittle fibers or whiskers. At ends or fractures in the fibers orwhiskers, the ductile matrix will flow locally and distribute the loadin the terminating fiber or whisker to adjacent fibers or whiskers,again over a reasonably long length of fiber or Whisker to preventthereby propagation of the fracture to neighboring fibers or whiskers.In this way, the potentially very high strengths of refractory ceramicmaterials is approached in realization of the invention.

Strengths of ceramic whiskers and fibers have been measured to be ashigh as 4X 10 p.s.i. Table 1 lists representative values:

TABLE 1 Pounds per square inch Graphite whisker 0.35 to 3.0x 10 A1 0whisker 0.16 to 2.60 10 BeO whisker 2.0 to 2.8 l0 SiC whisker 0.26 to1.50 l0 B whisker 096x10 W wire 040x10 SiO fiber 0.20 10 It has beenobserved that strengths of composites may be interpolated between thestrengths of the matrix and the fiber or whisker up to approximately 50percent fiber or whisker content but additional fiber or whiskeradditions do not increase composite strength. It is desirable withplatinum to use high contents of fibers or whiskers in order to reducethe cost of the composite. The difliculty in producing high fiber orwhisker content composites lies in the problem of maintaining acontinuous matrix in contact with all fiber or whisker surfaces at fiberor whisker contents. Unless each fiber or Whisker is completelysurrounded by matrix, it cannot be properly loaded. It is possible toproduce fully bonded composites with fiber or Whisker contents as highas 80 percent by applying the platinum metal to individual fibers orwhiskers before consolidation. This may easily be done by suspending thefibers or whiskers in an aqueous platinizing solution and controllingthe metal content by the weight increment on the fibers. Alternately, itmay be accomplished by vapor deposition of the platinum metal onto thefibers or whiskers.

With the possibility of using a variety of fibers or whiskers having anappreciable range of strengths as shown in Table 1, it is desirable tobe able to control the matrix strength as well in order to develop theoptimum balance between components. This may be accomplished by alloyingwithin the family of platinum-like metals. In the following reference isshown the mechanical properties of a family of platinum-rhodium alloys:Hill, J. S., Hot Tensile Properties of Platinum and Its Alloys,Englehard Industries, Inc., Technical Bulletin, June 1962, volume III,No. 1. Other platinum-type alloys are also feasible.

The reinforced platinum metal matrix composites of the present inventionwith fiber or whisker contents of from 1 to 80 volume percent canobviously be prepared according to any one of a number of conventionalart techniques such as, for example, those disclosed in Sutton et al.,Metals Engineering Quarterly, supra.

If, however, the intended application is such that a simple stresspattern exists, appreciable benefit can be derived by orienting thefibers or whiskers with the principal stress axes. As disclosed incopending application, Ser. No. 600,165, filed of even date herewith,now Pat. No. 3,432,295, this can be accomplished by preparing thecomposite as a mixture of discontinuous fibers or whiskers, matrixplatinum metal or alloy in the form of powder, and an organic bindersuch as polyvinyl alcohol. If this mixture is extruded with a large arearatio, 100:1 or higher, e.g., extruded through a die provided with anorifice whose diameter is less than the lengths of the reinforcingfibers or whiskers, a high degree of fiber or whisker orientation isobtained within the extruded rods. This material can then be cut tolength, and a large number of short sections staked into a hot pressingdie in a parallel configuration, and a dense body with a high degree oforientation produced by hot pressing. A 10 volume percent A1whisker-platinum metal matrix composite prepared in this manner by hotpressing at 1300 C. and under 2 t.s.i. for two hours, followed bysintering at 1250 C. in air for two hours, exhibited a tensile strengthof 38,000 p.s.i., as compared to approximately 20,000 p.s.i. withoutwhiskers. In like manner, there can be prepared additional materialcombinations of platinum matrix and 20, 30, 40 and 50 volume percent,respectively, of a whisker material as named above including A1 0 In aspecific instance, four such additional combinations were made using SiOwhiskers. In the event that a biaxial or a triaxial stress state existsin the application, a biaxial or triaxial fiber orientation can also beprepared. In the event that a high fiber or whisker content is requiredsuch that all of the matrix metal is applied to the fibers or whiskersbefore consolidation, it is possible to achieve a similar orientation byadding sutficient organic filler to the coated fiber or whisker toprovide a suitable plastic mass for extrusion, removing the organicmatter by thermal decomposition during final consolidation. It isapparent that numerous other deformation and consolidation processes canbesubstituted forextrusion and hot pressing without changing thesubstance of the invention.

As additional reinforcing fibers or whiskers contemplated, reference ismade to those disclosed in the hereinbefore mentioned copendingapplication.

The strength obtainable at elevated temperatures is superior to allcompetitive metals. Oxidation resistance is of major importance in mosthigh temperature applications, and the refractory metals, which offerthe only potential strength competition to the newly inventedcomposites, are notoriously poor in this respect. In the followingreferences are shown the relative oxidation rates of the refractorymetals as a class and the platinum metals as a class: Jaif, R. I.,Refractory Metals, High Temperature Technology Conference, October 1959,Stanford Research Institute; and Phillips, W. L., Jr., Oxidation ofPlatinum Metals in Air, Trans. ASM 1964, 57, pp. 33-37. It is concludedthat improvements of 5 or 6 orders 0 magnitude are possible.

As many apparently widely different embodiments of this invention may bemade without departing from the spirit and scope thereof, it is to beunderstood that the same is not to be limited to the specificembodiments thereof, except as defined in the appended claims.

What is claimed is:

1. A high strength, high ductility and oxidation resistant compositestructure, comprising a matrix selected from the group consisting ofplatinum and a platinum alloy, and a reinforcing component selected fromthe group consisting of A1 0 fibers and A1 0 Whiskers dispersed withinsaid matrix.

2. The composite structure of claim 1, wherein the matrix is platinum.

3. The composite structure of claim 1, wherein the matrix isplatinum-rhodium.

4. The composite structure of claim 1, wherein the reinforcing componentis oriented.

References Cited UNITED STATES PATENTS 3,005,876 4/1963 Alexander29182.5 XR 3,282,658 11/1966 Wainer 29l83.5 3,421,862 1/1969 Shyne 205XR OTHER REFERENCES Sabunas: Metal Fiber Composites in ProductEngineering, 53060, pp. 57-61.

CARL D. QUARFORTH, Primary Examiner A. J. STEINER, Assistant ExaminerUS. or. X.R. 29 1s2.5, 191.2; 75 200, 206

